Crystalline perovskite materials in the family of BaCeO3 and SrCeO3 and their doped analogues are well recognized candidates for utility as proton conducting electrolytes in solid oxide fuel cell (SOFC) applications. BaZrO3 and SrZrO3 have also received attention for use as SOFC electrolytes. Gadolinium doped or yttrium-doped BaCeO3 and BaZrO3 show particular promise due to their high proton conductivities. Much work has been reported for the preparation of these materials by conventional ceramic processing techniques including mixed oxide methods, chemical co-precipitation, and various solution-based approaches including the sol-gel and Pechini methods and variations thereof. Solid sintered bodies as well as thick films and thin films of these materials are of interest.
The solid state reaction-based mixed oxide method of ceramic synthesis requires repeated and extended mechanical grinding and calcination steps. As is typical of the mixed oxides approach, it is not possible to easily obtain very fine grained, homogeneous, single-phase perovskites of high purity in the BaCeO3, SrCeO3, or BaZrO3 families. Since the principal driving force for sintering of ceramic powders into the high density solids required for fuel cell application is surface energy, preferred ceramic powders have very small particle size that in addition to having high surface energies, also enable relatively high densities in their pre-sintered green state. In this aspect, both the agglomerated particle size and the crystallite size are factors impacting the pre-sintered green density. It is not sufficient that crystallite size be small to drive sintering to the high densities at the lowest sintering temperatures. This is particularly the case when crystallites in the green state are assembled into aggregates having strong chemical bonding between crystallites. Very small particle size for Y-doped BaCeO3 has been reported in “A New Combustion Process for Nanosized BaCe0.95Y0.05O3-δ Powders” JOURNAL OF RARE EARTHS, Vol. 22, No. 5, October 2004, p. 658, by Meng Bo et al. Although the authors disclose single phase material having a size of the order of 40 nanometers (nm), elimination of agglomerates formed by their process is only achieved after mechanical milling. Most references for the preparation of BaCeO3 materials report particle sizes of 100 nm up to micrometer-sized particles.
A significant disadvantage of the combustion process referenced above and similar approaches is that the organic materials required for the chelation or complexing of metal ions must be burned away as part of the process of forming the desired ceramic powders. The undesirable features include the consumption of costly materials and the production of waste gasses including carbon dioxide.
What is needed is a process for preparing doped or undoped BaCeO3, SrCeO3, BaZrO3, and SrZrO3 that gives rise to single-phase perovskite powders having crystallite size below 40 nm and with limited aggregation of crystallites, using relatively low process temperatures, and wherein the process does not produce significant waste.